This invention relates to stereotactic devices and localization methods used in neurosurgical operations in human patients. It more particularly relates to a method which utilizes CT/MRI based anatomical reference points or fiducial markers to localize a defined entry and target point and proper trajectory in three-dimensional space. It also specifically relates to a guidance system and instrument holder to hold and to guide ventricular catheters as well as other needed surgical instruments in a precise orientation until the instrument tip is placed into the intracranial target point.